\begin{tabular}{lcccccc}
\hline\hline     \\[-1.8ex]
& \multicolumn{2}{c}{Assignment} & \multicolumn{2}{c}{Survey response} & \multicolumn{2}{c}{Rate}       \\
\cline{2-7} \\[-1.8ex]
& \shortstack{Individual\\feedback}    & \shortstack{General\\feedback}    & \shortstack{Individual\\feedback}    & \shortstack{General\\feedback}   & \shortstack{Individual\\feedback}    & \shortstack{General\\feedback} \\
\hline \\[-1.8ex]
First feedback round   &       65 &       67 &       55 &       55 &     84.6\% &     82.1\%  \\
Second feedback round  &       64 &       67 &       56 &       55 &     87.5\% &     82.1\%  \\
Third feedback round    &       65 &       72 &       57 &       62 &     87.7\% &     86.1\%  \\
Forth feedback round    &       65 &       71 &       49 &       51 &     75.4\% &     71.8\%  \\
\hline\hline
\end{tabular}